From the past, batteries such as lead-acid batteries and the like have been used as power supplies for various types of device. Dilute sulfuric acid is filled into these batteries as battery fluid.
FIG. 11 is a figure showing an example of an uninterruptible power supply which employs a battery. The schematic structure of this uninterruptible power supply is that it comprises a rectification circuit 2 which is connected to the commercial AC power supply 1, a switching circuit 3 which is connected to the output side of the rectification circuit 2, and a plurality of batteries 10 in series, likewise connected to the output side of the rectification circuit 2. The switching circuit 3 is a device which converts the output voltage of the rectification circuit 2 or the voltage of the batteries 10 by switching to AC voltage which it outputs, and this output is supplied as driving electrical power to a load 4.
When the commercial AC power 1 supply is available as normal, along with the output voltage of the rectification circuit 2 being inputted to the switching circuit 3, the batteries 10 are also charged up by this output voltage of the rectification circuit 2. However, when an interruption occurs in the supply of power from the commercial AC power supply, the batteries 10 discharge, and their discharge voltage is inputted to the switching circuit 3. Due to this discharge, the supply of power to the load 4 is maintained.
The batteries 10 are housed in a case 11, so that even if, hypothetically, one of the batteries 10 should suffer liquid leakage, then the battery fluid 12 which has thus leaked out does not flow out from the case.
However, if the case 11 is made from metal, then the pole plates within the battery 10 and the case 11 are electrically connected together via the battery fluid 12 which has thus leaked out, and, as shown by the broken lines with arrows in the figure, a short circuit is created via the commercial AC power supply 1, the rectification circuit 2, the battery 10, and the case 11. When this short circuit is created, a large current flows through the portion which has become electrically conducting due to the battery fluid 12, and sparking may occur, which is undesirable.
If the case 11 is made from resin, then it is at least possible to avoid electrical conduction between the pole plates in the battery 10 and the case 11 taking place via the battery fluid. However, even if the case 11 is made from a resin, nevertheless, if liquid leakage has occurred from a plurality of the batteries 10, then a short circuit between these batteries 10 is undesirably set up via the battery fluid 12 which has leaked out, and a large current flows in the portion which has become electrically conducting due to the battery fluid 12, and, again, undesirable sparking may occur.
As a technique for detecting leakage of battery fluid, there has already been proposed, in Japanese Patent Laid-Open Publication 2003-243047, a battery fluid liquid leakage sensor which comprises, in superimposed layers, a first insulating member which is pervious to battery fluid, a first electrically conductive member which is pervious to battery fluid, a second insulating member which is pervious to battery fluid, a second electrically conductive member, and a third insulating member which is impervious to battery fluid.